**Core Concept**
In radical neck dissection, the primary goal is to remove all lymph nodes and surrounding structures on the same side of the neck to treat or prevent the spread of cancer. This procedure involves removing various anatomical structures, but some are spared to preserve vital functions.

**Why the Correct Answer is Right**
The structure not removed in radical neck dissection is the _sternocleidomastoid muscle_ (SCM). This is because the SCM is a vital muscle for neck movement and stabilization, and its removal could lead to significant functional impairment. The SCM is also an important landmark for surgical dissection in the neck.

**Why Each Wrong Option is Incorrect**
**Option A:** The _internal jugular vein_ is often removed during radical neck dissection to ensure complete removal of lymph nodes and to prevent recurrence of cancer. Therefore, it is not the correct answer.
**Option B:** The _spinal accessory nerve_ (cranial nerve XI) is usually sacrificed during radical neck dissection, as it is often involved with or near the lymph nodes to be removed. Its removal can result in shoulder dysfunction.
**Option C:** The _submandibular gland_ is not removed during radical neck dissection, but this is not the best answer. The correct answer is the SCM.

**Clinical Pearl / High-Yield Fact**
Remember the mnemonic "SCM is Safe" to recall that the sternocleidomastoid muscle is spared in radical neck dissection.

**Correct Answer: C. Sternocleidomastoid muscle**